Question

$C_2{O}_4^{2-}$ is a weak ligand but acts as a strong ligand in $K_3\left[Co\right(C_2{O}_4{)}_3]$ and induces the pairing of electrons against Hund's rule. Why?

Answer 1

For the complex $K_3\left[Co\right(C_2{O}_4{)}_3]$

Let consider the oxidation state of $Co=x$

Charge on oxalato ion $=-2$

Charge on Potassium ion $=+1$

$3(+1)+x+3(-2)=0$

$x=6-3$

$x=+3$

So, the oxidation state of Cobalt is +3. When central metal is present in +3 oxidation state, oxalate ligands acts as strong ligand and cause pairing of electrons of d - orbital which leads to the formation of the inner orbital complex with $d^{2}s{p}^3$ hybridization.